Waveform-shaping circuit and a data-transmitting apparatus using such a circuit

1. A waveform-shaping circuit, which level-discriminates an input signal so as to carry out a waveform-shaping operation by using the higher of the two values, that is, a first threshold value that is selected as a peak value of a first signal that varies in response to the input signal and a second threshold value that is selected as the average of the input signal, comprising: a peak-hold circuit for finding the peak value of the first signal; an average-value calculating circuit for finding the average of the input signal; and a capacitor that is commonly used for a holding operation in the peak-hold circuit as well as for an integrating operation in the average-value calculation circuit, wherein: the direction of currents to be supplied to the capacitor from the peak-hold circuit and the average-value calculating circuit is limited only to a first direction and the waveform-shaping circuit is further provided with a bypass means, installed in parallel with the capacitor, for allowing current to flow in a direction reversed to the first direction.

2. The waveform-shaping circuit as defined in claim 1, further comprising: a level shift circuit for reducing the level of the input signal by a predetermined value and for outputting the resulting signal to the peak-hold circuit as the first signal.

3. The waveform-shaping circuit as defined in claim 1, further comprising: an amplifier for amplifying the input signal so as to generate an inversion output and a non-inversion output; a level-shift circuit for reducing the non-inversion output by a predetermined value; and a voltage-dividing circuit for voltage-dividing the difference between the output of the level-shift circuit and the inversion output by a predetermined ratio and for outputting the resulting voltage to the peak-hold circuit as the first signal.

4. The waveform-shaping circuit as defined in claim 1, wherein the peak-hold circuit comprises a comparator which supplies an electric current in the first direction when the value of the first signal is higher than the output of the peak-hold circuit.

5. The waveform-shaping circuit as defined in claim 4, wherein the comparator comprises: a differential amplifier having a non-inversion input terminal to which the first signal is applied and an inversion input terminal to which the output terminal of the capacitor is connected; and a first rectifier that is installed between the differential amplifier and the output terminal of the capacitor.

6. The waveform-shaping circuit as defined in claim 1, wherein the peak-hold circuit, the average-value calculating circuit, the capacitor and bypass means are integrated onto an integrated circuit.

7. The waveform-shaping circuit as defined in claim 1, wherein the average-value calculating circuit comprises: a second rectifier for limiting an electric current to be supplied to the capacitor to the first direction; and a low-pass filter whose cutoff frequency is set not more than a frequency to which the second rectifier is allowed to respond, the low-pass filter being placed between the input terminal of the input signal and the second rectifier.

8. An infrared data-transmitting apparatus for receiving data through infrared light that is spatially transmitted, comprising: a photoelectric transfer element for converting the spatially transmitted infrared light into an electric signal; and a waveform-shaping circuit, which level-discriminates the input signal so as to carry out a waveform-shaping operation by using the higher of the two values, that is, a first threshold value that is selected as a peak value of a first signal that varies in response to the input signal derived from the electric signal and a second threshold value that is selected as the average of the input signal, comprising: a peak-hold circuit for finding the peak value of the first signal; an average-value calculating circuit for finding the average of the input signal; and a capacitor that is commonly used for a holding operation in the peak-hold circuit as well as for an integrating operation in the average-value calculation circuit, wherein: the direction of currents to be supplied to the capacitor from the peak-hold circuit and the average-value calculating circuit is limited only to a first direction and the waveform-shaping circuit is further provided with a bypass means, installed in parallel with the capacitor, for allowing current to flow in a direction reversed to the first direction.

9. The infrared data-transmitting apparatus as defined in claim 8, wherein the waveform-shaping circuit further comprises: an amplifier for amplifying the input signal so as to generate an inversion output and a non-inversion output; a level-shift circuit for reducing the non-inversion output by a predetermined value; and a voltage-dividing circuit for voltage-dividing the difference between the output of the level-shift circuit and the inversion output by a predetermined ratio and for outputting the resulting voltage to the peak-hold circuit as the first signal.